Steam generator heated with liquid metal

ABSTRACT

A compact liquid-metal heated vapor generator (21) in which the liquid metal is typically sodium and the secondary fluid is water. The water is conducted through tubes (29) of serpentine configuration with the serpentine turns (51-53) nested. The tubes (29) are in modular bundles (27) and the modules extend throughout the interior of the pressure vessel (23). The sodium flows countercurrent to the secondary fluid, through the vessel, penetrating between the tubes in intimate heat-exchange relationship with the tubes. The tubes (29) are sealed through tube sheets (37), (39); the upper tube sheet (39) above the level of the sodium and the lower tube sheet under a stagnant or quiescent zone (41) of sodium.

[ Oct. 30, 1973 STEAM GENERATOR HEATED WITII LIQUID METAL [75] Inventor: William F. Stahl, Media, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: June 10, 1968 [21] Appl. No.: 735,598

3,338,301 8/1967 Romanos 165/163 X 3,398,789 8/1968 Wolowodiuk et al 165/158 X FOREIGN PATENTS OR APPLICATIONS 1,096,388 l/l96l Germany 165/163 Primary Examiner-Albert W. Davis, Jr. Attorney-A. T. Stratton, F. P. Lyle and F. Cristiano, Jr.

[57] ABSTRACT A compact liquid-metal heated vapor generator (21) in which the liquid metal is typically sodium and the secondary fluid is water. The water is conducted through tubes (29) of serpentine configuration with the serpentine turns (51-53) nested. The tubes (29) are in modular bundles (27) and the modules extend throughout the interior of the pressure vessel (23). The sodium flows countercurrent to the secondary fluid, through the vessel, penetrating between the tubes in intimate heat-exchange relationship with the tubes. The tubes (29) are sealed through tube sheets (37), (39); the upper tube sheet (39) above the level of the sodium and the lower tube sheet under a stagnant or quiescent zone (41) of sodium.

6 Claims, 15 Drawing Figures PAIENIEDBBI 30 ms SHEET 10F 5 INVENTOR William F. Stohl WITNESSES i PAIENTEI] nm 3 0191s SHEET 2 OF 5 PATENIEUBBI 30 I975 SHEET 3 [1F 5 A W F OOOOOOO FIG. l5.

PAH NIHlnm 30 ms SHEET 40F 5 oo o STEAM GENERATOR HEATED WITH LIQUID METAL BACKGROUND OF THE INVENTION This invention relates to vapor generators and heat exchangers for such generators and has particular relationship to generators of the liquid-metal heated type and'heat exchangers for such generators. A vapor generator typically includes a pressure vessel through which primary and secondary fluids are conducted in heat-exchange relationship. The secondary fluid is converted into vapor by the heat derived from the primary fluid. In liquid-metal heated generators the primary fluid is a liquid metal such as mercury, sodium, an alloy or mixture of sodium and potassium, alloys, of, sodiumbismuth, bismuth-lead and others. The liquid metal derives its heat from a primary source of heat, typically a nuclear reactor. The secondary fluid is usually conducted through tubes in the pressure vessel. The primary fluid flows through the pressure vessel over these tubes. The word vapor" is used in this document with the understanding that it includes within its scope various fluids produced under supercritical as well as subcritical temperature and pressure conditions.

In liquid-metal-heated vapor generators in accordance with the teachings of the prior art the secondary fluid is conducted through helical tubes mounted or suspended about a conduit (see Schlichting et al. U.S. Pat. No. 3,112,735, Schlichting Pat. No. 3,256,932 tubes 40A, 40B, 40C FIG. ,3, 735 or 40 FIG. 6, 932). In accordance with other teachings of the prior art the secondary fluid tubes are intricate serpentine involutes, nested in banks, about a central column (see Enrico Fermi Atomic Power Plant EFAPP). These prior-art generators have the deficiency in common that they are not compact. For power output of 1,000 or 2,000 megawatts, a vapor generator in accordance with this teaching of the prior art would be so large that it could not readily be handled. The cost of the prior-art liquidmetal-heated generators is also high, in fact, so high that it is doubtful that this prior art lead to economically feasible apparatus. It is also desirable that a liquidmetal heated vapor generator be provided which has the utmost reliability so that the cost, to the users, of shutdown for repairs is minimized or entirely eliminated. I

It is an object of this invention to overcome the deficiencies of the prior art and to provide a reliable, relatively low-cost compact vapor generator of the liquidmetal-heated type, particularly for turbine-driven electrical power generating plants in the thousand megawatt range which shall be capable of being readily handled for transportation or other purposes.

SUMMARY OF THE INVENTION ln accordance with this invention, a liquid-metalheated vapor generator is provided in which the secondary fluid is conducted through a plurality of tubes in serpentine configuration extending, tightly packed, but spaced sufficiently to permit large volume flow of primary fluid, throughout the interior of the pressure vessel. The packing should be as close as feasible taking into consideration the fact that the liquid metal has a low specific heat or heat capacity so that a large volume of liquid metal is required to supply the necessary heat for vaporizing the secondary fluid. Typically the specific heat of liquid sodium is about 7 calories per gram atom per centigrade degree or about 0.3 calories per gram for centigrade degree. Other metals have like low heat capacity.

While other contours are feasible, it is advantageous in the interest of compactness that each serpentine unit be planar and includes a plurality of tubes mutually nested together. A plurality of such units are combined into a modular bundle of tubes and the modular bundles are mounted throughout the vessel. The tubes are sealed through a tube sheet associated with a header at each end of the vessel. So that effective heat exchange may be achieved in a compact vessel the transverse arms of the serpentines are long. The tubes conducting the secondary fluid then have high effective lengths in the region in which the secondary fluid is heated by the primary fluid.

Typically the generator is of the once-through type. The vessel is mounted with the tubes extending vertically and the fluid flowing upwardly from a feed-fluid header to a vapor header above which the vapor is derived. The liquid-metal primary fluid, typically sodium, is conducted through lined pipes near the top of the vessel to a manifold whence it is distributed throughout the vessel flowing countercurrent to the secondary fluid. The primary fluid flows over each of the tubes and between the, tightly packed, serpentine units in intimate heat-exchange relationship with the tubes. The tubes are composed of a single material which is resistant to the primary fluid (typically lncoloy 800) and which conducts the heat to the secondary fluid effectively (thickness of about l/l6 inch). The vessel is composed of stainless steel which is resistant to the liquid-metal primary fluid (typically AISI 316 stainless steel). The liquid-metal primary fluid having a low specific heat (sodium is about 0.3) and gives up its heat readily to the secondary-fluid tubes. Since the interior of the vessel is tightly packed heat losses are minimized.

The secondary-fluid inlet or feed tube sheet is disposed below a zone of quiescent primary fluid in which the disturbances are suppressed by baffles. The quiescent zone protects the seals of the tubes in the lower inlet tube sheet against rapid transient temperature changes so that the seal joints may be of the ordinary low-cost type and still be highly reliable. The seals may also be of the type disclosed in FIG. 5 of the Schlichting patents.

Typically the vessel is of generally circularly tubular configuration. The tubes extend axially throughout the interior of the vessel perpendicular to the generally circular cross-section. Modular bundles of serpentine tube units of different dimensions are provided for mounting in difi'erent regions of the cross section of the vessel. The tubes are all of the same effective length with regard to active heat transfer from inlet tube sheet to outlet tube sheet throughout the cross section of the vessel; the axial spacings between the turns of the serpentine being such as to achieve this purpose. Where the length along a linear branch of a serpentine is smaller the spacings between turns of the serpentine is reduced. The modular bundles may also be so formed that they are all alike throughout the cross section of the vessel. In this case all tubes are of the same length and have the same spacing between turns of the serpentine.

The liquid-metal heated vapor generator according to this invention has a number of advantages:

1. It is compact and of low total weight.

2. It can be constructed with great flexibility and full thermal shielding to provide service reliability.

3. It lends itself to use of a single tube material thus reducing manufacturing difficulty and costs.

4. The heat transfer is once-through and this simplifies the apparatus and reduces costs.

5. The liquid metal (sodium) flow is inherently stable thus eliminating baffles and achieving a very low liquidmetal pressure drop.

6. The apparatus can be constructed so that it is maintainable with both secondary and primary fluid sides completely drainable.

7. A unit of apparatus having a capacity of 1,000 megawatts may be shipped by rail to most areas in the United States.

Typically, a 1,000 megawatt generator with liquid sodium as the primary fluid has many significant advantages. It is a high performance compact unit which requires a minimum of foundation and maintenance access space. It has a specific power density of about 900 kw/cb ft., compared to no more than 100-200 for other steam generators currently in service or being designed. The small size not only reduces the cost of the 1,000 megawatt unit, but decreases installed costs because of lower weight, less space requirements and less stored-energy to provide for. This high performance results from the selection of a single, high strength tube material, Incoloy-800, for use in, typically, 34; inch OD tubes bent into a serpentine configuration and closely packed into asymmetrical, modular geometry. This generator possesses excellent heat transfer characteristics clue to high steam and sodium heat transfer and the use of low resistance, thin-walled tubing. Provision for thermal expansion is readily made with the tube support design which allows for expansion and slip without changing the shell-side flow or heat transfer geometry. The generator has spherical tube sheets (a feature of this invention) which are technically sound, and provide for significant improvement in operation. This feature allows a considerable reduction in tube sheet thickness and weight while providing for simpler, stron ger means of fabrication and assembly.-

The generator has a very low shell side pressure drop, and the sodium flow, being downwardly during cooling, is inherently stable. The modular structure of the heattransfer area is particular worthyof notice. All the tubing within the steam generator is subdivided into five module types, each of which is repeated in each quadrant of the generator. In this arrangement, the sodium flow redistribution, both within a single nodule and among the different modules must be assured.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a view in perspective, with a part of the wall broken away, showing a vapor generator constituting a preferred embodiment of this invention;

FIG. 2 is a view of the vapor generator of FIG. 1, partly in section and partly diagrammatic, showing the important features of this generator;

FIG. 3 is a view in cross-section, partly diagrammatic in the interest of clarity, taken along line IIIIII of FIG.

FIG. 4 is an enlarged fragmental view of the part of the generator shown in FIG. 1 which is in circle IV of FIG. 3;

FIG. 5 is an enlarged view in elevation of a section of a serpentine tube bank or modular bundle for the secondary fluid and the support of this bank or bundle;

FIG. 6 is a view in transverse section, enlarged, through a tube bank or tube module, as indicated at line VI-VI in FIG. 5, showing the support for the tubes conducting the secondary fluid;

FIG. 7 is a fragmental view in longitudinal section, enlarged, showing the thermal shield for the exit tube sheet, through which the tubes conducting the secondary fluid are sealed;

FIG. 8 is a fragmental plan view of this thermal shield where the tubes are arranged in a triangular pattern with the tube centers located at the apexes of equilateral triangles;

FIG. 9 is a view similar to FIG. 8 but showing the thermal shield for apparatus in which the tube centers are located at the apexes or rectangles;

FIG. 10 is a view similar to FIGS. 8 and 9 where the centers of the tubes are located at the apexes of triangles that are not equilateral;

FIG. 11 is a view, enlarged, of the portion of the apparatus in circle XI of FIG. 2 showing the liner for the pipe conducting the primary fluid;

FIGS. l2, l3 and 14 are fragmentary views in section, enlarged, showing joints or seals of different types for the secondary-fluid tubes through the tube sheets; and

FIG. 15 is a view in transverse section of a modification of this invention in which the tube bundle modules are alike throughout the cross-section of the generator shell.

DESCRIPTION OF PREFERRED EMBODIMENTS The apparatus shown in the drawings is a oncethrough vapor generator 21 of the vertical-shell-andtube type. The apparatus includes a generally cylindrical pressure vessel 22 provided with upper and lower hemispherical channel head portions 23 and 24 composed of corrosion-resistant material (typically AISI 316 stainless steel, where the primary fluid is sodium) within which there is an assembly 25 of modular tube bundles 27 each including a plurality of tubes 29 of planar serpentine configuration nested as shown in FIG. 2; with the radially more outward turns of the serpentine serving to nest adjacent radially more inward turns nearer to the periphery of the vessel 22 and the same radially more outward turns themselves nesting in the adjacent radially more inward turns nearer the center of the vessel 22a. The vessel 22, has lined liquid-metal inlet conductors or nozzles 31 which are generally diametrically spaced and feed liquid metal to a manifold 33 within which is disposed a foraminous spreader plate 34 (FIG. 1) that distributes the metal substantially uniformly over the tube assembly. The vessel also has liquid-metal outlet nozzles or conductors 35 also substantially diametrically disposed which conduct the primary fluid to heaters (not shown) for recirculation through the inlet conductors 31.

The tubes 29 are sealed through a generally spherical feedwater tube-sheet 37 at the base of the vessel 22 which forms a water chamber 38 with the lower channel head 24. The tubes 29 are sealed at their upper ends through a spherical tube-sheet 39 at the top of the vessel, which forms a steam chamber 40 with the upper channel head 23. Because the tube-sheets 37 and 39 and the channel heads 23 and 24 are spherical, their thicknesses may be minimized. Liquid to be vaporized is admitted to the chamber 38 by an inlet 38a (FIG. 1) and vapor is withdrawn from the upper chamber 40 by a suitable outlet 40a. The tube-sheet 37, as illustrated, is below the level of the primary-fluid outlet conductors 35 so that a quiescent zone 41 (FIG. 2) of liquid metal collects over the tube-sheet 37. The liquid in this zone 41 tends to be stagnant and circulation is suppressed by baffles 43. There is also a relatively quiescent zone 45 (FIG. 2) of liquid metal above the surface of the spreader plate 34 where the primary fluid is projected by the manifold 33 over the tube assembly. The effective length of the tubes 29 is that length lying in effective heat transfer relation to the sodium stream, i.e., that between the line 47, just below the manifold 33 and the line 49 just above the outlet conductors 35.

Because the liquid-metal flows downwardly in the shell 22, the flow is stable and does not tend to become stratified. The secondary fluid flows upwardly through the serpentine shaped tube assembly 25 in a multipass cross-flow pattern. Many passes are employed to approach thermodynamically ideal counter flow.

The total required heat exchanger capability for typically a 1,000 megawatt generator 21 is obtained in a single unit as shown in the drawings. The economics of large nuclear central stations are greatly improved by the use of single units of large thermal capacity. The once-through type of heat exchanger is simple in construction and operation; it is of lost cost, and of optimum compactness and has ready adaptability to eventual super-pressure steam conditions, and it has very low water retention in event of leakage. Its compactness allows complete shop assembly, shipment and installation as an integral unit. For a steam generator, either under sub or super critical conditions, with sodium as primary fluid, the tube material (INCOLOY-SOO), is available which is compatible with both liquid metal and water systems. This eliminates the very difficult manufacturing problems arid major expense associated with bimetal or double wall tubes. Carbon steel may be used for the feedwater tube sheet 37 where water temperatures of 600F exist. Since heat transfer rates are relatively high on both sodium and steam sides of the tubes 29, the tube wall conduction resistance is a major factor in determining the amount of transfer surface required. By using small diameter tubes typically inch, the wall thickness required to contain the pressure is also relatively small, typically (0.042 inch minimum). This aids in keeping the required surface area to a minimum and results in a small bundle volume. The relatively small tubes 29 also limit the rate at which water could leak into the sodium should a tube break occur. Provisions are made for very rapid drainage of water and if necessary sodium drainage as well.

In the apparatus shown in the drawings the tubes 29 are spaced closely together, yet provide between them the relatively large sodium flow areas required for a moderate sodium pressure drop. The attendant compactness of the bundle due to this close tube spacing is reflected in the small overall size of the unit and correspondingly thinner shell walls. In turn, this feature enhances the generators ability to withstand thermal transients.

So that the fluid may be effectively vaporized, it is essentially that the arms 51 of the serpentines into which tubes 29 are formed transversely of the axis of the vessel 22 be as long as practicable. These arms 51 are long compared to the axial arms 53 (FIG. 5). Typically the relationship is such that effective length of the secondary fluid tubes 29 is about six times the length of the active zones 47 to 49 in which the heating takes place, and at least greater than three times said length.

The tube modules 27 extend throughout the cross section of the vessel 22. Since the vessel 22 is of circular cross-section transversely to the flow of the primary fluid, the modules 27 are of different width in different parts of the cross section. Typically there are modules 27 labelled A, B, C, D, E (FIG. 3) of five different widths. Modules of width A extend across the cross section at the ends. These modules are abutted by modules of width E which also extend across the cross section between the peripheries of the vessel 23 on both sides. The latter modules are abutted by modules of width A which extend from the central vertical plane 61 of the vessel 23 to the periphery on each side. The modules of width B are abutted by modules of width C, in turn, abutted by modules of width D, all of which extend from the center 61 to the periphery. The modules of width D are each abutted at the center by modules of width E which also extend from the center 61 to the periphery.

The tubes 29 in all modules are essentially of substantially the same length. The spacing between the arms 51-53 of the serpentines of the narrower modules A, B,

C must then be smaller than the corresponding spacing for the wider modules D and E. The thickness of the modules are different (FIG. 3) and are selected so that the modules fit into the cross section.

Alternatively, modules 63 (FIG. 15) of like dimensions may be provided. In this case each module is typically in the form of a parallelepiped with the length axial of the vessel 23 perpendicular to the bases and the bases parallelograms with one set of opposite angles 120 and the other set 60. The tubes 65 are arranged with a parallelogram pitch to form skewed modules. The modules 63 are disposed in abutting groups in 120 sectors of the cross section of the vessel 23.

The modular tube assembly 25 is encased in a tubular wrapper 71 concentric with the casing 22 (FIG. 3), and the modules are separated by tube support sheets 73 (FIGS. 4 and 6).

The tubes 29, as best shown in FIG. 5, are supported by notched elongated strips or plates 75 extending vertically and transversely to the tubes and attached to cross rods 77 traversing the tube banks or modules 27. The cross rods 77 in turn, are supported by and tie together, the different support sheets 73. The support plates 73 carry the weight to the wrapper 71 which, in turn, is supported from the shell 22 at a plurality of locations.

The wrapper 71 encases the entire tube assembly and connects at the bottom to the sodium outlet pipe liner 81. The assembly 25 is supported through the wrapper 71 at the bottom to the shell 22. The expansion movement between the wrapper 71 and the inlet pipe liners 79 is readily accommodated by the clearances.

The pipe liners 79 and 81 which conduct the liquid metal through the nozzles 31 and 35 are shielded thermally by generally cylindrical shield structures 91 (FIG. 11). The shields 91 protect the external pipe wall 82 from severe thermal stresses by reducing the rate of heat flow to or from the wall (79 or 81). The shields 91 also reduce the relative thermal expansion between the pipe wall and the liner by reason of transients acting in the region where the liner is fixed and thus improves the mechanical integrity of the apparatus in this region.

The shield structures 91 are of generally cylindrical form coaxial with the pipe liners 79. The shield structures 91 each include a sleeve 93 which abuts the pipe wall 82 closely and is secured to the wall by a weld 95 around its rim. A generally cylindrical heat' shield 97 is suspended from the sleeve 93 by an intermediate radial member 99 and aligned with the liner wall 79. There are additional shields 101 between the shield 99 and the sleeve 93.

The shield 91 lies in a region surrounded by relatively stagnant liquid metal (except for possible eddies) which leaks back through the wrapper 71. The eddies are reduced by radial members 103.

Although suitable shutoff and control features can alleviate the stresses produced by operating transients, emergency situations such as a reactor scram or a loss in feedwater or sodium flow are likely to be so severe as not to be alleviated this way. For example, the steam generator liquid-metal inlet temperature can rapidly dropalmost 500F following a reactor scram. This would result in a similar drop in steam exit temperature and a surge of water flow into the steam 40. A water pump failure would result in a rapid 500F increase in steam generator sodium exit temperatures.

In the generator according to this invention constraint between adjacent thermally expanding or contracting metal parts are reduced to the point where thermal stresses are sufiiciently low to meet long life requirements. The serpentine tube itself represents a highly-flexible structure design afforing adequate feasibility to permit thermal expansion. Tube-support hardware is also free to expand, as are various baffles and wrappers. This hardware is light, and is not subject to thermal shock damage. Intersecting walls between the nozzles 31, 3S and shell 23 are blended where necessary to alleviate thermal stress difficulties. Suitable thermal isolation is provided at tube-to-steam-tube sheet joints. Typically temperature changes in the primary fluid produce severe thermal stresses in the tube sheets 37 and 39. Such stresses arise particularly because the tubes themselves respond (expand, or contract or are deflected) rapidly to primary (or secondary) fluid temperature changes and create stresses in their header connections. The stresses also arise from the circumstance that the portions of the tube sheets 37 and 39 immediately adjacent the tubes 29 respond more rapidly than the other portions, creating the stresses in the channel heads.

The stress created by the response of the tubes 29 themselves is alleviated by providing a flexible connection between the tubes 29 and the tube sheets 37 and 39. Such a flexible connection is achieved by a trepanned annular groove 111 (FIG. 12) around each tube-to-tube-sheet welded vacuum-tight joint 1 13. Like flexibility is provided by an omega-type insert 115 sealed around the welded joint 113 (FIG. 13) by an annular vacuum-tight weld 114; or by extending the tube 29 beyond the tube-sheet (FIG. 14) and employing a sleeve 117 extending around theprojecting portion of the tube 29 and joined to the tube 29 and tube-sheet 39 by vacuum-tight welds 119 and 121, respectively.

The stress produced by the rapid response in the portions of the tube sheet adjacent the tubes is alleviated by thermally insulating the tube-sheet 39 so that the tube-adjacent portion does not respond rapidly to changes in temperature. The insulation must be with respect to both the fluid inside of the tubes and also from any fluid that may become deposited on the tube sheet 39. The tube-adjacent portion of the tube sheet may be insulated from the fluid in the tubes 29 by providing an annular clearance space 123 about the tubes 29 (FIGS. 12 and 14) or an annular stagnant region 125 (FIG. 13) between the main body of the tube sheet 39 and the end of the tubes 29.

The protection against the deposit of hot unvaporized secondary fluid on the upper tube sheet 39 is afforded by the apparatus shown in FIGS. 7, 8, 9, 10. The tube-sheet 39 has tubular projections 131 through which the tubes 29 extend. The tubes 29 are welded by vacuum tight welds 133 to the rise of the projections 131. The tubes 29 extend above the projections 131 and a plurality of blocks 135 which are slipped over the tubes 29 and projections 131 and are nested as shown in FIG. 8 for example.

Where the tubes 29 are centered at the apexes of equilateral triangles, the blocks 135 are of hexagonal outer-cross-section (FIG. 8). Where the tubes 29 are centered at the apexes of rectangles or squares, the blocks 141 (FIG. 9) are of square or rectangular outercross section, respectively. Where the tubes 29 are centered at the apexes of non-equilateral triangles, the blocks 143 are of rectangular outer cross section and dimensioned appropriately so that they nest. In any event steam or super-heated water (or other fluid) emitted from the tubes 29 is deposited on the blocks and thus the tube-sheet 39 is protected against rapid thermal transients.

In regions where a high degree of constraint is unavoidable, thermal shielding is provided to reduce the rate of change of temperature of each protected surface to a level where thermal stresses are not severe. Such shielding is necessary on all portions of the shell 22, tube-sheets 37, 39 and nozzles 31, 33 in contact with the liquid metal. Typically, liquid sodium is a good heat-transfer medium and thermal shock to these heavier metal parts would be severe without such shielding.

A highly purified and highly pressurized inert gas (argon or helium typically) is inserted in any suitable manner and occupies the space (FIG. 2) about the manifold 33 and above the liquid-metal level 151 and acts as a cover gas. Metal components in contact with this cover gas respond slowly to liquid-metal temperature changes and should not require thermal shielding. The process of heat transfer from liquid metal to gas to metal by natural convection is relatively slow. The water side of the water tube-sheet 37 does not experience any severe transients except for a sudden decrease in pressure following scrams. This is similar to transients occurring in present conventional power plants.

The lower portions of the tube modules 27 are formed as indicated at C (FIG. 2) to closely fit in the circular area of the lower tube sheet 37. The tube-sheet 37 is protected by a plurality of thermal shields 43 (FIG. 2) maintained in spaced relation with each other by spacers 43a. These shields set primarily as fluid baffles to create a stagnant or quiescent pool or zone 41 of liquid metal between the flowing metal and the tube sheet 37. The most severe transient condition for the tube sheet 37 would be the stoppage of secondary flow while liquid metal flow continues. Heat would then flow from the metal in the outlet region downwardly through the stagnant pool 41 to the tube sheet 37. Since the heat flow is downwardly, there would not be any tendency to create convective fluid currents and the heat would be transmitted by thermal conduction alone. While the thermal conductivity, typically of sodium, is very high with respect to other common fluids, it is only about three times as high as for stainless steel. The thermal conduction through the pool, which is about two feet thick, shields the tube sheet, which may be 2.5 inches thick, against any severe thermal stress. Holes 152 are preferably provided in the baffles 43 to |prevent pressure buildup from a tube or tube-plate leak linto the liquid-metal. Any hydrogen evolved vents :through the baffles, thence through the shell shielding and up to the top of the generator 21.

Shielding in the form of stagnant or quiescent liquid metal between the wrapper 71 (FIGS. 2 and 4) and the shell 22, protects the shell walls from rapid transients. Typically, 6 inch thickness is allowed for shielding. The

shell shielding follows principles similar to that of the water tube sheet 37, i.e., it utilizes the thermal conduction resistance and heat capacity of the liquid metal itself to decrease the heat transmission. Since the shell is vertical, there is a convection tendency during transients. This can be inhibited by breaking up the annular sodium zone 153 (FIG. 2) of the shield 71 into a cellular matrix by a second shield 154 encompassing the wrapper 71.

There are two zones of liquid metal in the shielding zone 153, an annular inner zone 155 and an annular outer zone 156. Hot inlet metal flows downwardly in the inner shield zone 155 and is cooled by heat transfer to the main metal flow through the wrapper 71 of the tube bundle or assembly 25. It then constitutes leakage into the main sodium flow at .thebottom of .the steam generator21 (at tubes 29 and at shell nozzles 35).

The outer shield sodium zone 156 contains quiescent sodium in temperature equilibrium with the inner zone, both zones may be subdivided to prevent convection. The extent and type of subdivision depends upon proven effectiveness and manufacturing economy. The shell 22 is preferably completely shielded with specially shaped pieces (not shown) fitted carefully around all nozzles 31, 35 and irregularities in contact with the metal. Provisions for draining and venting the shielding spaces are incorporated.

The upper portions of the tube modules 27 are converged inwardly and upwardly as indicated at D (FIG. 2) to conform to the cross-sectional area of the upper tube sheet 39 and are encompassed by a tubular shield 160. The tube-sheet 39 is also shielded by annular shielding strips 161 and spaced baffles 163 disposed withirr the tubular shield 160.

TABLE 1 Unit Type Once-Through, Counter Multi-pass Cross-Flow, Vertical, Shell and Tube Unit Weight 200,000 lbs. SHELL SIDE GEOMETRY Fluid contained Sodium Outside Dia in. Wall Thickness 2.3 in. Overall Length 34 ft. Flow Baffles None Design Temperature 1200F. Design Pressure 300 psig Type 316 S.S.

Material TUBE SlDE GEOM ETRY Fluid Contained Water and Steam Tube Material lncoloy-800 Number of Tubes 6000 Outside Diameter 0.375 in.

Wall Thickness (avg.) 0.046 in.

Efi'ective Length 59 ft.

Installed Length 71 ft.

Pattern Serpentine Pitch 9/16 in. between tube centers with flow; 5/8 in. between tube centers across flow Design Temperatures l200F Design Pressure 2500 psig.

TUBE SHEET GEOMETRY Type of Tube-Sheet Spherical Thickness of Water Side 2.5 Thickness of Steam Side 1 1 The steam generator 21 according to this invention, wherein the liquid metal is sodium, typified by the above tabulation, is constructed to operate in a vertical position. The various internal parts are well supported when the generator is in that position. But the parts may not be well supported if the generator were in a horizontal position. Since it is necessary to place the generator in a horizontal position for some manufacturing processes and especially for shipping, some means must be provided for temporarily bracing the tube bundle 25 and other parts inside the shell 22. For this purpose the shell 22 of the generator may be filled with liquid sodium which would be allowed to solidify, thus securely holding the tube bundle 25.

The sodium is in the present apparatus highly satisfactory as a potting material since the materials of construction of the generator 21 must all be compatible with sodium and the generator must be cleaned, filled with inert gas, and otherwise in a condition suitable to receive sodium. The same sodium can be used in the operation of the steam generator 21.

Suitable manway covers 180 and 181 may be provided and attached to the lower and upper channel heads 24 and 23, respectively, for gaining access to the associated chambers, for servicing purposes, as well known in the art.

Also an emergency outlet 185 (FIG. 2) may be provided at the upper end of the vessel 22 and closed by a rupture disc 1 86. In the event of sudden overpressure in the vessel 22, the disc 186 will rupture and release pressurized fluid from the vessel for discharge through the pipe 188.

While preferred embodiments of this invention have been disclosed herein many modifications thereof are within the spirit of the invention. This invention then is not to be restricted except as is necessitated by the prior art.

I claim as my invention:

1. A liquid-metal heated vapor generator, comprising a vertically extending pressure vessel having a tubular wall portion and upper and lower spherical end portions,

an upper and lower tube sheet cooperatively associated with said upper and lower spherical end portions and jointly therewith defining an upper vapor chamber and a lower liquid chamber, respectively,

a plurality of tubes extending substantially throughout the interior of said vessel in a planar serpentine configuration and having their upper and their lower ends sealed through said upper and lower tube-sheets, respectively, and in fluid communication with said vapor chamber and said liquid chamber,

means for admitting a heated liquid metal to said vessel,

means for conducting said liquid metal about said tubes in good heat-exchange relation with said serpentine configuration, and establishing a body of vapor as it issues from said upper-tube-sheet intosaid vapor space, said tubes also being nested together in closely spaced relation to each other in a direction transverse to the direction of flow of the liquid metal,

said tube nest being encompassed by a tubular wrapper disposed in internally spaced relation with the tubular wall portion of the vessel,

a tubular shield encompassing said wrapper and jointly with said wrapper and the vessel wall portion forming two annular spaces for stagnant liquid metal, and

means for withdrawing said pressurized vapor from said vapor space.

2. The structure recited in claim 1, wherein means including a plurality of vertically spaced baffles are disposed above the lower tube sheet to restrict circulation of liquid metal therepast and form a stagnant pool of liquid metal.

3. The structure recited in claim 2, wherein the liquid metal outlet is disposed above the baffles to promote formation of the stagnant pool.

4. The structure recited in claim 3, and further including a baffle structure disposed beneath the upper tube sheet to inhibit splashing of hot liquid metal against the tube sheet.

5. The structure recited in claim 1, and further including a tubular liner encompassing the tubes,

the tubes being arranged in a plurality of modules of rectangular cross-sectional shape nested in a manner to substantially fill the cross-sectional area within said wrapper,

the upper portions of the tubes extending from the serpentine configuration and converging toward the upper tube sheet,

a tubular baffle structure disposed beneath the upper tube sheet and encompassing the upper converging portions of the tubes.

6. The structure recited in claim 1 and further includa plurality of vertically spaced baffle plates disposed within the tubular baffle,

the upper tube portions extending through the baf fies, and

said baffle plates and the tubular baffle being effective to inhibit splashing of hot liquid metal against the associated tube sheet. 

1. A liquid-metal heated vapor generator, comprising a vertically extending pressure vessel having a tubular wall portion and upper and lower spherical end portions, an upper and lower tube sheet cooperatively associated with said upper and lower spherical end portions and jointly therewith defining an upper vapor chamber and a lower liquid chamber, respectively, a plurality of tubes extending substantially throughout the interior of said vessel in a planar serpentine configuration and having their upper and their lower ends sealed through said upper and lower tube-sheets, respectively, and in fluid communication with said vapor chamber and said liquid chamber, means for admitting a heated liquid metal to said vessel, means for conducting said liquid metal about said tubes in good heat-exchange relation with said serpentine configuration, and establishing a body of liquid metal in said vessel, means for withdrawing said liquid metal from said vessel, means for admitting a pressurized liquid to be vaporized to said liquid chamber, means defining a gas space above said body of liquid metal, means for admitting a pressurized inert gas to said gas space, said tubes being of sufficient length to permit conversion of said pressurized liquid into pressurized vapor as it issues from said upper-tube-sheet into said vapor space, said tubes also being nested together in closely spaced relation to each other in a direction transverse to the direction of flow of the liquid metal, said tube nest being encompassed by a tubular wrapper disposed in internally spaced relation with the tubular wall portion of the vessel, a tubular shield encompassing said wrapper and jointly with said wrapper and the vessel wall portion forming two annular spaces for stagnant liquid metal, and means for withdrawing said pressurized vapor from said vapor space.
 2. The structure recited in claim 1, wherein means including a plurality of vertically spaced baffles are disposed above the lower tube sheet to restrict circulation of liquid metal therepast and form a stagnant pool of liquid metal.
 3. The structure recited in claim 2, wherein the liquid metal outlet is disposed above the baffles to promote formation of the stagnant pool.
 4. The structure recited in claim 3, and further including a baffle structure disposed beneath the upper tube sheet to inhibit splashing of hot liquid metal against the tube sheet.
 5. The structure recited in claim 1, and further including a tubular liner encompassing the tubes, the tubes being arranged in a plurality of modules of rectangular cross-sectional shape nested in a manner to substantially fill the cross-sectional area within said wrapper, the upper portions of the tubes extending from the serpentine configuration and converging toward the upper tube sheet, a tubular baffle structure disposed beneath the upper tube sheet and encompassing the upper converging portions of the tubes.
 6. The structure recited in claim 1 and further including a plurality of vertically spaced baffle plates disposed within the tubular baffle, the upper tube portions extending through the baffles, and said baffle plates and the tubular baffle being effective to inhibit splashing of hot liquid metal against the associated tube sheet. 